Mucosal infection with HIV-1 is characterized by the rapid induction of type 1 interferons (IFNs) at the initial sites of entry. However, the extent to which these cytokines control HIV-1 replication during the earliest stages of infection and their contribution to the transmission bottleneck are not understood. We recently discovered that transmitted founder (TF) viruses that have crossed the mucosa and initiated a productive systemic infection are significantly more resistant to the antiviral effects of type 1 IFNs than viruses that predominate during chronic HIV-1 infection (1, 2). These findings strongly suggest that antiviral genes up-regulated by type 1 IFNs during the earliest stages of infection exert significant selective pressure on the transmitted HIV-1 pool, resulting in the establishment of systemic infection by variants that are relatively IFN resistant. In this application, we propose t capitalize on this observation by identifying the IFN-stimulated antiviral genes that counteract HIV-1 replication at the site of entry and by determining whether these effector mechanisms can be exploited for the design of new prophylactic and therapeutic strategies to combat HIV-1. Our working hypothesis is that understanding the host antiviral effector mechanisms which control HIV-1 during the earliest stages of infection will lead to new interventions that are capable of impairing virus acquisition and initial spread, and may also have therapeutic utility. We have established a novel virus-based approach that will allow us to dissect the contribution of type 1 IFNs to the early anti-viral state in the mucosa and characterize the particular interferon stimulated genes (ISGs) involved, along with the TF virus determinants that confer resistance to their activity. Specific Aims are: 1. To quantify the impact of type 1 IFN resistance on HIV-1 transmission fitness. 2. To examine the potential of different type 1 IFN subtypes to restrict HIV-1 transmission. 3. To map the viral determinants that confer type 1 IFN resistance on transmitted founder viruses. 4. To identify the IFN-stimulated genes (ISGs) that exert selective pressure on HIV-1 during the earliest stages of infection. We expect these studies to reveal whether type 1 IFNs have the capacity to extinguish foci of virus replication at the mucosal site of entry and to identify the particular interferon-stimulated genes (ISGs) that mediate this important early antiviral activity.